1. Field of the Invention
The present invention relates to a fine-adjustment mechanism for a scanning probe microscopy for scanning a sample by adjusting relative positions of a probe and the sample with a fine-adjustment mechanism for performing measurement of roughness or physical properties of a sample surface, machining of the sample surface, or movement of a substance on the sample surface by the use of a probe.
2. Description of the Related Art
In a fine-adjustment mechanism of a scanning probe microscopy in the related art, for example, a cylindrical piezoelectric device as shown in FIG. 8 and FIG. 9 is employed (for example, see Patent Publication No. 2598665, pp. 4, FIG. 1). FIG. 8A is a cross-sectional view of a cylindrical piezoelectric device, and FIG. 9 is an expansion plan of an outer peripheral surface. In this related art, a common electrode 82 is provided inside a piezoelectric device 81 formed into hollow cylindrical shape, and an outer electrode is formed into a two-level structure, including a band-shaped electrode 83 extending along the circumference thereof on a first level and an electrode 84 which divides the circumference into four parts on the second level.
The band-shaped electrode 83 of the cylindrical piezoelectric device 81 is applied with a poling treatment uniformly from the inner electrode 82 to the outer electrode 83, and the quadrant electrode 84 is applied with the poling treatment so that two sets of electrodes 84a, 84c and 84b, 84d located on the opposite sides with respect to a center axis have opposite polarities from the inner electrode 82. Arrows indicated in FIG. 8A show poling direction at each electrode, and plus and minus signs in FIG. 9 represent polarities of poling in the direction of outer peripheral surface.
When a potential difference is provided between the inner electrode 82 and the band-shaped electrode 83 on the first level from a power source 83, a strain in the direction of thickness is generated in the area where the band-shaped electrode 83 is provided along the entire circumference, and consequently, the cylindrical piezoelectric device 81 is displaced toward the direction of the center axis (hereinafter referred to as Z-direction).
When voltage is applied between the two opposing electrodes 84a, 84c and 84b, 84d of the quadrant electrode 84 provided on the second level from the power supplies 86, 87, one of the electrodes is expanded in the axial direction and the other electrode is contracted in the axial direction as shown in FIG. 8B, whereby the cylindrical piezoelectric device is bent with respect to the axial direction. When the distal end 81a of the cylindrical piezoelectric device is fixed as a fixed end and the proximal end 81b of the same is remained free as a free end, the distal portion 81b is displaced into an arcuate shape in the direction indicated by an arcuate arrow in FIG. 8B. When the angle of strain is minute, the distal end moves approximately within a plane orthogonal to the center axis. Therefore, displacement within a two-dimensional plane (hereinafter referred to as X-Y direction) is achieved by the quadrant electrode 84. Normally, the quadrant electrode 84 is provided on the fixed end side, and is configured in such a manner that the output displacement is increased by an amount corresponding to the length of the band-shaped electrode 83.
In the scanning probe microscopy, measurement of roughness or physical properties on the sample surface, machining of the sample surface, or movement of a substance on the sample surface by the probe are enabled by providing the sample or the probe at the distal end of the cylindrical piezoelectric device and causing the sample or the probe to scan in the X-Y direction while controlling the distance in the Z-direction.
By configuring the fine-adjustment mechanism for the scanning probe microscopy with the cylindrical piezoelectric device configured as described above, the following advantages are achieved.
(1) With the piezoelectric device, fine-adjustment with high degree of accuracy on the order of sub-nanometer is achieved.
(2) By forming the piezoelectric device into cylindrical shape, rigidity of the device unit is increased, and by combining fine-adjustment mechanisms in X, Y, and Z directions into a unit, the entire device can be downsized, and rigidity of the device can be enhanced. Consequently, anti-vibration property or scanning speed can be improved.
However, when moving the probe and the sample with respect to each other by the piezoelectric device, non-linear movement is resulted due to hysteresis or creeping characteristic of the piezoelectric device, thereby generating displacement error.
Therefore, a method of compensating by calculation after having obtained a hysteresis curve of the piezoelectric device or a method of measuring displacement of the piezoelectric device directly by a displacement sensor are employed.
Therefore, when compensating the error by calculation, since there arise various parameters such as variations in machining among devices, the peripheral temperature, the range of scanning, the speed of scanning, and the direction of scanning as factors which determine the action of the piezoelectric device, it is difficult to establish a compensation formula considering all these parameters, whereby sufficient accuracy of measurement cannot be achieved.
When integrating a displacement sensor on the other hand, a capacitance type displacement sensor, an optical displacement sensor, a linear variable differential transformer displacement sensor, or an eddy-current displacement sensor is used. However, these sensors are all bulky and hence a large space is required for installation, thereby increasing the size of the entire device. Consequently, rigidity of the device is decreased, whereby resolution or scanning speed is deteriorated. The cost of the displacement sensor itself is also high.
A method using a strain gauge is know as a method which can be implemented in a small space and enables easy detection of displacement (for example, see “Revised New Edition: Approach to Measurement of Strain Using a Strain Gauge”, attributed to Sho Takahashi, Masayasu Kawai, published from Taiseisha, Jun. 1, 2001, pp. 63, pp. 95-pp. 97))
In general, a metallic resistor is used in the strain gauge. A problem found in a case where the strain gauge is used is that a high resolution cannot be obtained due to deterioration of S/N ratio when measuring fine strain, and that the value of resistance varies with temperature significantly, whereby the magnitude of error of measurement increases.
Therefore, in general, a bridge circuit is configured with four resistors for compensating temperature and amplifying the output.
FIG. 10 is a drawing showing a bridge circuit used for strain measurement. The amount of strain of the resistors R1 to R4 are represented by ε1 to ε4, and the output voltage e when a voltage E is applied to the circuit is represented by an expression below.e=Ks/4·(ε1−ε2+ε3−ε4)·E  (1)
where Ks is referred to as gauge factor, which is specific for the type of the strain gauge. When configuring a bridge circuit, it is necessary to select the strain gauges of the same gauge factor.
When detecting strain in the direction of compression/extension, a bridge circuit is configured with the strain gauges mounted as shown in FIG. 11 to FIG. 15. In the following description, strain generated in the device is represented as ε, and temperature strain generated on the gauge due to variation in temperature is represented as εT. Wiring to the strain gauges is not shown.
FIG. 11 refers to a Single-Gauge Method. FIG. 11A shows a method of adhering the strain gauge, and FIG. 11B shows a wiring method of the strain gauge. As shown in FIG. 11A, a strain gauge 112 is adhered on a member 111 which is expanded and contracted in the direction of the center axis as indicated by an arrow and generates compression/extension strain. The strain gauge 112 is formed with a resistor 113 in the direction of the center axis, whereby a bridge circuit as shown in FIG. 11B is configured. The resistors other than R1 are fixed resistors. The amount of strain of the fixed resistors is zero, the output voltage is:e=Ks/4·(ε+εT)·E  (2)from the expression (1). Therefore, in the case of Single-Gauge Method, the temperature compensation by the bridge circuit cannot be performed.
FIG. 12 refers to a Two-Gauge Method. FIG. 12A shows a method of adhering the strain gauges, FIG. 12B shows a wiring method of the strain gauges. As shown in FIG. 12A, the strain gauges 122, 123 are adhered to a member 121 which is expanded and contracted in the direction of the axis as shown in the arrow and generates compression/extension strain in such a manner that the resistors 124, 125 are oriented in the direction of the center axis and the direction orthogonal to the direction of the center axis, whereby a bridge circuit as shown in FIG. 12B is configured. Reference numerals R3, R4 represent fixed resistors having the same value of resistance as the strain gauges. In this case, when Poisson's ratio of material is represented by v, and strain in the direction of center axis is represented by ε, the expressione=Ks/4·(1+v)ε·E  (3)is satisfied. In other words, the temperature compensation is achieved, and the output voltage is amplified to 1+v times.
FIG. 13 also shows Two-Gauge Method. FIG. 13A shows a method of adhering the strain gauges and FIG. 13B shows a wiring method of strain gauge. In this case, as shown in FIG. 13(a), the strain gages 132, 133 are adhered on a member 131 which is expanded and contracted in the direction of the center axis as indicated by an arrow and generates compression/extension strain in such a manner that the resistors 134, 135 extends in parallel with the direction of the center axis, whereby a bridge circuit as shown in FIG. 13B is configured. The resistors R2, R4 are fixed resistors having the same value of resistance as the strain gauge. The output voltage in this case is:e=Ks/4·2(ε+εT)·E  (4)
In other words, the output voltage is doubled, and the amount of strain due to the temperature is also doubled. Therefore, temperature compensation is not achieved.
FIG. 14 shows a method referred to as Four-Gauge Method. FIG. 14A shows a method of adhering strain gauges, and FIG. 14B shows a method of wiring the strain gauges. As shown in FIG. 14A, the strain gauges 142-144 are adhered on the member 141 which is expanded and contracted in the direction of the center axis as indicated by an arrow and generates compression/extension strain in such a manner that the resistors 146, 148 of the strain gauges 142, 144 are oriented in the direction of the center axis, and the resistors 147, 149 of the strain gauges 143, 145 are oriented in the direction orthogonal to the center axis, whereby a bridge circuit as shown in FIG. 14B is configured. The output voltage in this case is:e=Ks/4·2(1+v)ε·E  (5)
In other words, the temperature compensation is achieved, and the output voltage is amplified to 2(1+v) times.
FIG. 15 is a system referred to as Active-Dummy system of Two-Gauge Method. FIG. 15A is a method of adhering strain gauges, FIG. 15B is a wiring method of the strain gauges. As shown in FIG. 15A, the strain gauge 153 is adhered on the member 151 which is expanded and contracted in the direction of the center axis as indicated by an arrow in such a manner that the resistor 154 of the strain gauge 153 is oriented in parallel with the center axis, and the strain gauge 155 is adhered on another member 152 formed of the same member as the member 151 in such a manner that the resistor 156 is oriented in parallel with the center axis, whereby a bridge circuit as shown in FIG. 15B is configured. The resistors R3, R4 here are fixed resistors having the same value of resistance as the strain gauges. In this system, the strain gauge 153 of the device 151 which generates strain serves as an active gauge, and the strain gauge 155 which is adhered on the separate member 152 formed of the member of the same material as the device serves as a dummy gauge. When the active gauge and the dummy gauge are placed under the same environment, the amount of strains with respect to the temperature is nearly the same, and hence strain due to the temperature is cancelled, whereby the output voltage of the bridge circuit is:e=Ks/4·ε·E  (6)
In the case of the fine-adjustment mechanism using the piezoelectric device, the amount of strain is extremely minute. Therefore, when accuracy on the order of sub-nanometer is required as in the case of the scanning probe microscopy, measurement error due to the influence of the temperature of the strain gauge cannot be ignored, and hence temperature compensation must be performed.
In this case, it is essential to perform the temperature compensation by one of the above-described methods shown in FIG. 12, 14, or 15.
In addition, in order to obtain the accuracy on the order of sub-nanometer, the metal gauge is insufficient in output signal and low in S/N ratio, whereby required output signal cannot be obtained. Therefore, a strain gauge having a resistor formed of semi-conductor material, which has higher gauge factor Ks than the metal gauge is used.
The semiconductor gauge has a high gauge factor, but then comes under significant strain due to the temperature. Therefore, the temperature compensation by the bridge circuit is essential.
However, the semiconductor gauge is smaller in maximum allowable strain than the metal gauge. Therefore, when the semiconductor strain gauge is adhered on a curved surface, it is bent to a degree exceeding the maximum allowable strain, and hence measurement cannot be performed. Therefore, adhesion in the circumferential direction of the cylindrical piezoelectric device as shown in FIG. 12 or FIG. 14 is not possible, whereby the temperature compensation cannot be achieved.
In the active-dummy system shown in FIG. 15, it is necessary to place various subjects to be detected under the identical environment. In the case of the scanning probe microscopy, since the member to which the dummy gauge is adhered is placed under the identical environment to the fine-adjustment element, it is necessary to place the member at a position as close as possible to the fine-adjustment mechanism. However, when this space for installing the member for the dummy gauge is secured in the unit of the scanning probe microscopy, the entire device is upsized, rigidity is deteriorated, and measurement accuracy is degraded. When the member for the dummy gauge is disposed outside the unit, there arises a difference in environment of installation. In the scanning probe microscopy, even a small difference in environment affects significantly on the measurement accuracy. Furthermore, since the piezoelectric devices as material of the fine-adjustment mechanism vary in mechanical characteristics due to the machining method, it is necessary to even out the characteristics of member forming the portion on which the active gauge is adhered and the portion on which the dummy gauge is adhered as much as possible.
In order to solve the problems described above, the present invention provides a fine-adjustment mechanism for a scanning probe microscopy for fine-adjusting the relative position between a subject to be measured (sample) and a probe in order to measure roughness or physical properties of the subject to be measured by scanning the surface of the subject to be measured by the probe including: a piezoelectric device constituting the fine-adjustment mechanism; at least two electrodes for applying voltage to the piezoelectric device, at least one of the electrodes being used as a dummy electrode on which no voltage is applied and the other electrode being used as an active electrode that generates a strain on the piezoelectric device when applied with voltage; one or two resistors for detecting the strain at one or two positions on the active electrode; and one or more resistors provided on the dummy electrode, wherein the respective resistors on the active electrode and the dummy electrode are connected so that temperature compensation is performed by the resistor on the dummy electrode when strain detection is performed by the resistor on the active electrode to form a bridge circuit.
Also, two or more active electrodes are provided and the respective resistors of the active electrodes share the identical resistor for temperature compensation on the dummy electrode to configure a bridge circuit.
Furthermore, the fine-adjustment mechanism is formed of a hollow cylindrical piezoelectric device comprising a common electrode provided within the cylinder and two or more electrodes on the outside thereof, wherein at least one of the two or more electrodes on the outside is used as a dummy electrode.
Still further, the resistor is formed of a linear pattern of semiconductor and is arranged so that the longitudinal directions of the linear pattern of resistor provided on each of the active electrode and the dummy electrode extend in parallel with the direction of the center axis of the cylindrical piezoelectric device.